For example, video centering devices are known from the prior art that may determine individual or even all parameters of the eyes of a user and/or of the usage position of spectacles or of spectacles frames in front of the eyes of a user by capturing an image of the user, and possibly with the assistance of additional resources. The parameters of the eyes and/or of the usage position include, for example, the interpupillary distance of the user, the face form angle of spectacles or a spectacles frame worn by the user, the spectacle lens inclination, the shape of the spectacles frame, the corneal vertex distance of the system of spectacles and eye, the fitting height of the spectacle lenses etc.
The publication DE 10 2005 003 699 A1 discloses a stereo camera system having two cameras that respectively capture an image of a user with spectacles from different capture directions, from which three-dimensional user data are calculated. Video centering devices with a camera are likewise known which make chronologically offset exposures of the eye area of the of the user to be measured from different viewing angles. The exposures are subsequently oriented relative to one another with the assistance of a reference object in the image (for example a centering clip) so that a stereo camera system is likewise formed. Other solutions use only the projection captured with a camera in a view of a wearer of a [spectacles] frame on which a centering clip is mounted. Some parameters may be approximately determined via an assumption of values of the corneal vertex distance, an inclination difference between centering clip and mount, and a face form angle.
The parameters of the eyes and/or of the usage position of spectacles or a spectacles frame in front of the eyes of a user, said parameters being determined by means of a video centering device, may on the one hand be considered in the fitting and insertion of spectacles lenses in a spectacles frame; on the other hand, optimizations in the spectacles lens itself may therefore be made in order to adapt it to the wear position in the usage position.
Camera-based devices are also known in which the content on a monitor varies depending on the bearing and/or head position of the user in front of the monitor. An example of a device that is designed to test the visual acuity of a user in which the size of the vision test characters may vary depending on the position of the user is disclosed in DE 10 2014 009 459.
In the above camera-based devices, it is often important to obtain information about the position of an object captured by the camera (for example an eye of a user) relative to said camera, in particular about the distance of the object from the camera.